1. Field of the Invention
The present invention relates to a process for surface modifying support membranes and to new and improved membranes produced from such process. More particularly, the present invention relates to a process for depositing a uniform layer of a hydrogel onto essentially all of the surface area of a support membrane. In a particularly preferred embodiment, a uniform layer of an ion exchange hydrogel is deposited onto essentially all of the surface area of a porous hollow fiber membrane.
2. Prior Art
Membrane modifying processes disclosed heretofore involve depositing a layer of coating material on the membrane either by solvent evaporation or by irreversibly adsorbing the modifying polymer onto the support membrane. In the solvent evaporation process, a solution of the modifying material is applied to the membrane, excess solvent is removed from the membrane and the membrane is dried to remove the remaining solvent. In some cases a surfactant is added to aid in obtaining a more uniform distribution of modifying material. This results in a modifier on the membrane whose quantity is determined by the amount of solution retained by the membrane and the concentration of the modifier solution. In the solvent evaporation process, the distribution of a modifier on a membrane depends upon migration phenomena occurring during the evaporation process. The coating can be stabilized by cross-linking the modifying material after or during drying. Inherent limitations in these methods are formulating the modifying material to obtain a sufficiently thin, uniform distribution of polymer on the membrane and often having the modifying material concentrated at one surface of the membrane.
The irreversible adsorption process is disclosed in U.S. Pat. No. 4,794,002 and involves soaking the support membrane in a solution of the modifying polymer dissolved in an appropriate solvent for a period of time sufficient for the modifying polymer to adsorb onto the support membrane. The resulting modified membranes are then washed with excess solvent. This particular process is suitable for surface modifying a variety of polymeric support surfaces by irreversibly adsorbing thereon a suitable modifying polymer which possesses the desired chemical characteristics. For example, a fabric made of a polymeric material such as nylon can be surface modified to increase its ability to wet and thereby render clothes made of such a modified fabric more comfortable. Employing this process, one can surface modify and render substantially hydrophilic various laboratory and medical devices which often are made of hydrophobic polymeric materials which interact undesirably with protein material. The surface modifications can be further stabilized by cross-linking or can be derivatized in order to incorporate desired reactive groups, such as, for example, ion exchange groups, e.g., derivatized using chloroacetic acid. Membranes modified by this process have a monomolecular layer of the modifying polymer, as opposed to a pellicular hydrogel, adsorbed onto essentially all of the surface area of the membrane. However, because such membranes contain only a monomolecular layer, adsorption capacity is limited.
U.S. Pat. No. 4,618,533 discloses surface modified membranes having passivation coatings which membranes are prepared by wetting a polymeric porous membrane and coating directly onto the entire surface thereof a monomer grafted and cross-linked in situ onto the membrane. Such membranes are prepared utilizing high concentrations of a free radical initiator and heating the monomer-containing membrane for a short period of time, such as from 2 to 30 minutes, at a temperature between 60.degree. and 95.degree. C. Membranes surface modified under these conditions are often difficult to prepare with reproducible properties because the polymerization reactions are conducted such that only a portion of the initiator is reacted. The reactions are interrupted before completion to avoid completely filling the pores in the membrane with hydrogel which would thereby drastically reduce the liquid permeability of the membrane (see, for example, U.S. Pat. No. 4,917,793). Furthermore, support membranes prepared in this manner, i.e., directly coated by grafting and cross-linking an ionic or functional monomer thereto, have a limited amount of said monomer bound to the membrane. The utility of such membranes is therefor limited when its application depends upon the quantity of said monomer incorporated into the coating material. For example, the ion exchange adsorption of a protein depends upon incorporation of sufficient ionically substituted monomer onto said coating material.